£90 for a tiny plastic timer - I'm being ripped off?

The tiny plastic clockwork time clock on my Glow-worm xtrafast 120 has given up the ghost, I've been quoted £90 just for the part. As it doesn't work I've taken it apart, it's a cheap little plastic thing with a microswitch, the plumber who helped me to extract it is a good mate and even he thinks the cost is way OTT. Can anyone point me in the direction of a more reasonable parts supplier. Thanks